We are happy to announce the release of EteSync!
EteSync is an open source, secure, end-to-end encrypted and journaled personal information synchronization solution that currently only supports contacts and calendar synchronization on Android, but more platforms and information types are in the making. This means that only you are able to access your data, your data is safe, and you have access to its full change history. You accidentally deleted a calendar event with important information a month ago? No problem, you can just look the event up in your change history and restore it (data is currently retained by the UI to access it is in the works).
The client's code is based on the widely used DAVdroid, which means the client should be stable, even though it's only now released to the public. We've been using it and haven't encountered any issues.